This invention relates to the field of high-speed optical communication and, more specifically, to optical modulator drivers.
Modulated signals are key components in high-speed optical transmission systems. Laser output is modulated into high-speed light pulses that transmit voice, data and/or video signals over fiber-optic cables. The modulators convert continuous laser light output into pulses, which eliminates the need for demanding, high-speed performance from the laser and reduces its cost. The modulator driver circuitry receives an electrical non-return-to-zero (NRZ) signal as its input and converts the NRZ signal to a voltage that is applied to the optical modulator. The modulator varies the intensity of an optical beam in proportion to the voltage applied to it. Thus, the optical beam modulated by the modulator serves as the optical output signal.
Current optical RZ modulators typically comprise two stage modulators, with the first stage forming an optical train of RZ pulses and the second stage selectively suppressing some of the RZ pulses based on the input data stream. The major disadvantages of the existing solution are that two-stage modulators have twice the losses of single-stage modulators and two-stage modulators require twice the number of components to drive the two modulators, increasing the form factor and the cost of the system.
The invention comprises a method and apparatus for generating return-to-zero modulated optical signals. The invention generates modulated return-to-zero signals from input electrical signals in a single stage format, thus providing lower costs by reducing the number of components and providing lower insertion losses allowing for longer transmission distances.
In one embodiment of the present invention, a method for generating return-to-zero optical signals includes the steps of applying an electrical signal to the first input of a logic element, manipulating the output of the logic element such that the logic element produces a resulting electrical signal that logically transitions only when the input signal has associated with it a predetermined logic state, shaping the resulting electrical signal according to a desired RZ optical signal, amplifying the shaped electrical signal such that a maximum and a minimum signal level of the shaped electrical signal correspond to the adjacent optical power transfer zeroes of an optical modulator, and applying the amplified electrical signal as a driver input to the optical modulator.
In another embodiment of the present invention, an apparatus for generating return-to-zero optical signals includes a logic element for receiving electrical signals as a first input to the logic element, a manipulating element for manipulating the output of the logic element such that the logic element produces a resulting electrical signal that logically transitions only when the input signal has associated with it a predetermined logic state, a pulse shaping element for shaping the resulting electrical signal according to a desired RZ optical signal, an amplifier, for amplifying the shaped electrical signal such that a maximum and a minimum signal level of the shaped electrical signal correspond to the adjacent optical power transfer zeroes of an optical modulator, and an optical modulator for varying the intensity of an input continuous light source in proportion to a voltage applied to it by the amplified electrical signal.